This invention relates to methods of operating turbochargers and to a unique turbine housing design for a turbocharger.
Turbochargers are well known in the prior art and conventionally utilize a turbine wheel and a compressor impeller mounted on a unitary or common shaft, carried within respective housings therefor. The turbine housing includes a gas inlet and a gas outlet wherein there is provided, in the typical case, an internal scroll configuration as is well known to the art.
In the typical case the turbine housing defines an inlet, an outlet located radially interiorally from the inlet, a centrally located turbine wheel cavity, and a volute passageway encircling the turbine wheel cavity and extending with a diminishing cross sectional area to encircle the turbine wheel cavity. Exhaust gases from an engine are directed to the turbine housing through the volute or scroll passageway therein. Since the passageway diminishes in cross sectional area, the velocity of the exhaust gases through the turbine housing is maintained at a high level throughout the circumferance of the turbine housing. Thus, throughout the circumferance of the turbine the exhaust gases impinge at a high, relatively uniform velocity on the turbine blades to rotate the turbine wheel, which in turn causes rotation of the compressor impeller.
The compressor impeller compresses ambient air and/or air-fuel mixture to supply the same in a compressed state to the intake manifold of the internal combustion engine. The internal combustion engine typically includes a plurality of cylinders, the gas outlets from which are coupled together in banks or manifolds. The exhaust gas manifolds from the sets of cylinders are coupled to the inlet of the turbine housing.
The exhaust gas outlets from the cylinders of the internal combustion engines used in turbochargers are conventionally connected in two banks or sets of an identical number of cylinders. Initially the exhaust manifolds from both sets of cylinders were combined and directed in a single flow pathway through a radial inflow turbine housing of the type described.
One problem which initially existed in turbochargers was that overcompression of the air or air-fuel supply mixture being fed to the internal combustion engine sometimes occured. Accordingly, it became necessary to reduce the exhaust gas flow through the turbine housing so that a portion of the exhaust gases bypassed the turbine wheel. The diversion of a portion of the exhaust gases was achieved by providing the exhaust manifolds of each set of cylinders of the internal combustion engine with a wastegate valve. A selected portion of the exhaust gases from each of the sets of cylinders was thereby diverted prior to reaching the inlet of the turbine housing. Using such an arrangement, selective control of exhaust gas flow became possible so as to allow control of compression and prevent overcompression of air and fuel at the inlet to the internal combustion engine. However, such arrangements required duplicate valving systems. Also, it was necessary to locate the wastegate valves in proximity to the turbine housing, or form the wastegate valves as an integral part of the turbine housing. Due to limited space in that particular area of the turbocharger, placement, installation and maintenance of the wastegate valves became a problem.
Another problem which existed in the turbocharger art was that the exhaust from the several cylinders of the internal combustion engine in the turbocharger did not create a uniform pressure in the exhaust manifolds, but instead resulted in the release of hot, high pressure exhaust gases in sharply defined surges or pulsations. Such surges and pulsations occur because immediately following the power stroke of a piston in a cylinder of an internal combustion engine, the gas in the cylinder is still under a high pressure. As the exhaust valve initially opens, the major portion of the hot exhaust gases quickly flows or surges at high velocity into the exhaust manifold. Following the initial surge, and during the remaining period that the exhaust valve is open, the flow of exhaust gases from the cylinder is at a relatively low velocity compared to the initial surge. The expulsion of exhaust gases following the initial surge occurs due to normal piston movement which decreases the volume of the cylinder, rather than as a result of a pressure differential between the cylinder and exhaust manifold such as exists immediately prior to opening of the exhaust valve of the cylinder.
Pulsations and surges in the exhaust manifolds of the banks or sets of cylinders did not occur in unison. Rather, the surges of exhaust gases within each exhaust manifold tended to be interspersed with surges in the other manifold. At the junction of the two manifolds at the inlet to the turbine housing, the surges and pulsations in each manifold tended to create a back pressure in the other manifold. This reduced both the efficiency of operation of the internal combustion engine, and also reduced efficiency of operation of the turbine wheel, since instead of providing pressure only to the turbine wheel, the energy of the exhaust gases created a manifold back pressure which adversely affected the exhaust gas flow from the other set of cylinders.
Various attempts were made to eliminate this problem. In particular, the construction of the turbine housing was altered to include a divider at the inlet to the turbine housing chamber of the turbocharger. The exhaust manifolds of the two sets of cylinders did not meet in a junction at the inlet to the turbine housing, but rather were directed through separate flowpaths within the turbine housing. It was discovered that in a radial inflow turbine housing for use with turbochargers wherein the housing was of the type having a generally 360.degree. volute housing chamber receiving gas flow through an inlet passage and discharging gas flow progressively radially inwardly circumferentially around the chamber into a generally central turbine wheel location, it was advantageous to extend a divider in the passageway to divide the flow passageway over an arc of between about 180.degree. and 300.degree.. The length of the divider was governed by the number of cylinders of the internal combustion engine with which the turbocharger is used as well as the speed range of the engine. In general, the lower the number of cylinders of the internal combustion engine, or the lower the speed range of the engine, the greater was the benefit of a divided turbine housing. Thus, for internal combustion engines having a relatively few number of cylinders or operated at relatively low speeds, the housing chamber was divided throughout an arc of up to a maximum of about 300.degree.. Conversely, the greater the number of engine cylinders or the greater the engine speed range, the smaller was the effect of the housing chamber divider. It was advantageous with such engines to terminate the housing chamber divider closer to the 180.degree. minimum limit. A turbocharger turbine housing design having such a divider is described in U.S. Pat. No. 4,027,994, issued June 7, 1977.
Another advance in the art of the turbochargers which was developed was the relocation of the exhaust gas wastegate from the inlet of the turbine housing to a bypass exist port intermediate the inlet to the turbine housing and the terminus of a single volute passageway defined in the turbine housing to carry the exhaust gas from a conventional junction of a dual manifold exhaust system to the centrally located turbine wheel positioned within the turbine housing. The primary reason for relocating the position of the wastegate apparatus was to remove it from the area of the inlet of the turbine housing so as to permit ease of placement of the turbine housing within an engine compartment and to improve the efficiency of operation of the turbine wheel. Such an arrangement is described in U.S. patent application Ser. No. 376,492, filed May 10, 1982.
Until the present invention the prior art failed to recognize the advantages which could be achieved by constructing a turbine housing for a turbocharger both with an interior divider of scroll configuration bifurcating the passageway into axially adjacent passages over an arc of between about 180.degree. and 300.degree., and providing the same turbine housing with an exhaust port in communication with the passageway between the terminus of the divider and the outlet to the turbine wheel. Such an arrangement simplifies the wastegating of a divided exhaust system while maintaining a good turbine efficiency. Such a wastegating system also eliminates the requirement for a double valve or two separate valves for use with an internal combustion engine in a turbocharger having a divided exhaust system.